Insulated electrical conductors, i.e. insulated wires and cables, designed for medium to high voltage applications, are generally constructed of a metal core conductor having arranged coaxially around the core conductor, in the order named, an internal semi-conductive layer, a crosslinked olefin polymer insulation layer, an external semi-conductive layer, a metal shield layer and an outer protective sheath. Conventionally, the external semi-conductive layer is based on a composition containing an ethylene-ethyl acrylate copolymer or an ethylene-vinyl acetate copolymer, and carbon black. If it is desired to crosslink the external semi-conductive layer, an organic peroxide is added to the composition.
It is important, for the successful and rapid installation, repair or splicing of insulated electrical conductors that the external semi-conductive layer be strippable from the insulation layer. In order to provide compositions which have adequate adhesion, coupled with strippability, it has been proposed to chemically modify the polymers of the compositions and/or add various additives thereto. For example, it is known that the adhesion between the crosslinked olefin polymer insulation layer and the semi-conductive layer can be decreased by increasing the comonomer content, that is, the ethyl acrylate or vinyl acetate content of the ethylene-ethyl acrylate copolymers or of the ethylene-vinyl acetate copolymers, used in formulating compositions to be used as semi-conductive layers. It is also known that a desired level of strippability can be achieved by chlorinating the ethylene copolymers rather than by increasing the monomer content thereof.
As to additives, it is known that strippability of the semi-conductive layer from the crosslinked olefin polymer insulation layer can be improved by the addition of silicone oil, such as liquid dimethyl polysiloxane, to the composition of the semi-conductive layer.
These and other such measures, however, have not proved to be particularly effective. Compositions containing chlorinated ethylene copolymers exhibit inferior mechanical properties and poorer thermal stability when used as external semi-conductive layers of electrical power cables. Also, silicone oil is not completely compatible with ethylene copolymers and in time, oozes out of the compositions when used in amounts sufficient to improve strippability, generally in excess of 5 percent by weight. Furthermore, the addition of silicone oil, in amounts sufficient to improve strippability, degrades mechanical properties, particularly elongation, of the resultant compositions.
Stripping of the external semi-conductive layer from the crosslinked olefin polymer insulation layer has become a more pressing problem in view of more recent extrusion and "curing" techniques. According to recent extrusion technology, insulated electrical conductors are manufactured by coextrusion by which three layers, the internal semi-conductive layer, the crosslinked olefin polymer insulation layer and the external semi-conductive layer are extruded simultaneously, employing coaxial extruders, and subsequently cured in a single operation. In one aspect, this method of manufacture is advantageous in that it results in the close bonding of the three layers, eliminating partial delamination and void formation between layers, caused, during normal use, by flexure and heat. This, in turn, prevents corona deterioration and other insulation degradation. On the other hand, such a method of manufacture presents problems of strippability due to the high bond strength between the crosslinked olefin polymer insulation layer and the external semi-conductive layer, caused by formation of crosslinking bonds across their interface. Such high bond strength can extend time for repairs and, in some instances, can result in damage to the insulation layer during stripping operations.
As to the new "curing" technique, curing of crosslinkable layers is changing from the conventional "steam" cure to "dry" cure. "Dry" cure involves the use of an inert gas, such as nitrogen, heated to temperatures higher than temperatures attainable with steam. This new technique results in increased productivity of the insulated electrical conductors and eliminates the formation of microvoids in the insulation, which had been caused by diffusion of steam therethrough.
Although "dry" curing has eliminated the problem of microvoids and makes possible curing at higher temperatures, it has given rise to other problems of a serious nature. The semi-conductive, outer layers of the electrical conductors, which are directly exposed to the high temperatures of the dry cure process undergo thermal degradation, particularly in the case of layers of ethylene-vinyl acetate copolymers and ethylene-vinyl acetate-vinyl chloride terpolymers. Thermal degradation results in rapid deterioration of the properties of the insulated electrical conductors.